Improving the longevity of metals such as stainless steel at elevated temperatures is of critical importance. The service life of many alloys, including stainless steel, can be limited by the growth of damaging oxide scale that exfoliates (spalls) during operation and ultimately that contributes to component failure. Corrosion conditions at elevated temperatures exacerbate the material loss. The corrosion and damaging oxidation of steel and other alloys presents problems in numerous applications. For example, in power generation applications, typical examples of problems associated with such damaging oxidation include: (1) accelerated high-temperature fire-side corrosion associated with the presence of molten alkali-containing salts; (2) accelerated medium-temperature fire-side corrosion associated with the presence of a low oxygen activity environment and sulfur; and (3) steam-side oxidation of tubing, piping and valves in fossil fuel-fired boilers. There is an emerging pressure to increase the efficiency of fossil fuel power plants while at the same time meet stringent environmental regulations and ensure plant reliability, availability and maintainability, all at low cost. (Stott, Mater. Sci. Tech. 5:734 [1989]).
Steam temperature is one of the key factors that controls plant efficiency and the emission gas. Increasing the steam operating temperature and pressure will increase the plant efficiency while reducing emission gasses (Viswanathan, J. Mater. Eng. Performance 10:81 [2001]). The former provides the financial advantage by reducing the operating cost with increased power production, and the latter makes the process more eco-friendly by decreasing the emission of hazardous gases such as SO2, CO2, and NOx. For example, using 538° C./18 MPa (steam temperature/pressure) steam plant condition as a base, an efficiency increase of nearly 6% can be achieved by changing the steam condition to about 593° C./30 MPa. At 650° C. this would be as much as 8%. Ecologically, an increase in 1% efficiency of an 800 MW plant would lead to the lifetime reduction in CO2 approaching 1 million tons (Viswanathan, 2001, supra). Clearly, there is a need for improved corrosion and oxidation resistance of steel and other alloys in the power generation industry. The benefits of improved resistance of steel and other alloys to corrosion and damaging oxidation are not limited to the power generation industry. The benefits of improvements in operation efficiency and increased component service life apply across countless applications and industries in which such steel and alloy components are used.
The alloys used in high temperature systems must possess good mechanical properties along with resistance to corrosion and damaging oxidation. However, it is not easy for a single alloy to have all these properties and to still provide ease of manufacturing. Alloys are often treated, e.g., by vacuum heating, to produce a thin, protective oxide layer on the surface of the metal. Although iron, nickel and cobalt-base alloys have been considered for high temperature applications, metal oxides of these elements are not protective enough when the working temperature exceeds 550° C. The addition of other elements such as chromium, aluminum and silicon has improved their corrosion resistance because of the establishment of more protective oxide layers (Cr2O3, Al2O3 and SiO2). These oxide layers not only offer a better protection because of low growth rate, they also are effective barriers against ion migration (Atkinson & Gardner, Corros. Sci 21:49 [1981]). However, the threshold amounts of these elements required for the alloy to form a continuous protective oxide layer depends upon the alloying elements and application, and these additions to alloys can have a deleterious effect on the mechanical properties of the alloy. Although chromium has a lesser effect on mechanical properties than aluminum and silicon, iron alloys require approximately 12% chromium to form a continuous chromium oxide film in air and to therefore protect against damaging oxidation (Stott 1989, supra). However, in steam environment the amount of Cr required is around 25% to form a complete protective chromium oxide layer (Otsuka, Sumitomo Met. 44:30 [1992]).
There remains a need for surface treatments to and methods of providing such treatments to improve the performance and service life of steel and other alloys used in oxidating and corrosive environments.